Anode etching of aluminum foils with high-purity in an electrolytic solution containing chloride ions (Cl.sup.-) in order to form pits and to increase its surface area (A) has been used to prepare foils for use in aluminum electrolytic capacitors. After the treatment, anodization is conducted in the electrolytic solution to form a dielectric forming film (the thickness is t and the dielectric constant is .epsilon.). The value of the capacitance (C) of the electrode foil is obtained by the formula, EQU C=.epsilon.(A/t)
In order to increase the capacitance (C), the surface area should be increased and the dielectric forming film should be thinner. Various methods of producing dielectric foils have been examined. U.S. Pat. No. 5,120,404 discloses a method of anodization of aluminum foils at a low voltage of 150 V at most, in which the aluminum foils electrolytically-etched using a pulse current are anodized in an aqueous solution of ammonium adipate. As for medium or high voltage usage, aluminum foils which are electrolytically etched using a direct current are provided with a hydroxide film and treated to decrease the conductivity to be smaller than that for a low voltage usage, using phosphoric acid or boric acid electrolytic solution. The method of anodization is substantially the same as for the low voltage method.
FIG. 19 is a schematic view showing one such method. In the anodizing tank in FIG. 19, numeral 1 is a feeding roller to supply aluminum foil and also to supply a direct current. Numeral 2 is a direct current (DC) power source, 3 is an anodizing tank to anodize the aluminum foil, 4 is an electrode plate to supply current into the solution, 5 is an aluminum foil as a substrate, 6 is an electrolytic solution, 7 is a bottom roller to transport the aluminum foil, 8 is an anodized aluminum foil and 9 is a transporting roller to transport the anodized aluminum foil outside the tank. The anode of the DC power source 2 is connected to the feeding roller 1 made of metals like copper (Cu) or silver (Ag), while the cathode of the power source 2 is connected to the electrode plates 4 in the anodizing tank 3. The aluminum foil 5 is anodized by continuously traveling on the feeding roller 1 while passing between the two electrode plates 4 in the electrolytic solution 6 inside the tank 3. And the foil is reversed by the bottom roller 7 at the bottom of the anodizing tank 3, again travels between the two electrode plates 4 and goes out from the solution surface, then travels while contacting with the transporting roller 9.
Next, a method of feeding by a general charging tank is explained referring to FIG. 20. FIG. 20 shows the structure of a feeding tank. In this drawing, 10 is a direct current power source to supply a direct current. 11 is a feeding tank to supply a current to the aluminum foil 13. 12 are electrode plates dipped in the feeding solution 14. 15 is a bottom roller, and 16 is a transporting roller. The anode of the DC power source 10 is connected to the electrode plates 12 disposed in the feeding tank 11, and the cathode of the power source 10 is connected to the electrode plates 4 (Z) in the anodizing tank 3 (cf. FIG. 19). The current from the direct current power source 10 is provided to the aluminum foil 13 through the electrode plates 12 and through feeding solution 14.
However, according to the conventional method, oxide coating films are not formed on the aluminum foil 5 when the non-anodized aluminum foil 5 travels continuously and enters between the two electrode plates 4 in the anodizing tank 3. Therefore, a large rush current on the order of from 10.sup.2 mA/cm.sup.2 to 10.sup.3 mA/cm.sup.2 flows as a current density at the area A of FIG. 19. The value depends on the conductivity of the electrolytic solution and also the distance between the electrode plate and the Al film. Then, the current density decreases as the oxide coating film grows. In this case, when the large rush current flows, the surface of the aluminum foil having fine irregularities melts and thus the increased surface area from the etching decreases. When the large rush current flows, Joule's heat is generated and the temperatures of the aluminum foil 5 and the electrolytic solution 6 are raised. This results in a quantity of aluminum ions eluting from the surface of the aluminum foil 5, and the ions become hydroxides on the surface of the aluminum foil 5. If this aluminum hydroxide is taken into the oxide coating film of the aluminum foil 5 or adheres to the foil, the oxide coating film becomes thicker or the etch pits are sealed. Because of these factors including the above-mentioned surface-melting, sufficient capacitance cannot be obtained and the oxide coating film may be distorted. As a result, leakage current increases as time passes. Similar to the case of the anodizing tank 3, current concentrates in the area D shown in FIG. 20 in the feeding tank 1, and hydrogen gas is generated from the aluminum foil 13. As a result, the aluminum foil 13 is deformed.